1. Field of the Invention
The invention relates to high dielectric strength capability epoxy resins, utilizing epoxy chromium ionic bonding within chromium intercalated silicate material upon cure, to provide a high voltage epoxy resin matrix for the intercalated silicate. These resins can be used for a wide variety of insulation applications for generator stators and rotors. The high dielectric strength will allow its use as very thin insulation and permit low-cost dip coating or spraying procedures to be used.
2. Background Information
Mica, a group of silicates, such as KAl.sub.2 AlSi.sub.3 O.sub.10 (OH).sub.2 (Muscovite) or KMg.sub.3 AlSi.sub.3 O.sub.10 (OH).sub.2 (phlogopite), has been long been a key component of high voltage electrical insulation in electrical machines over 7 Kv, because of its particularly high dielectric strength, low dielectric loss, high resistivity, excellent thermal stability and excellent corona resistance. Presently, mica is used in the form of flakes on a glass fabric backing, which provides mechanical integrity required for machine wrapping of coils, as shown for example in U.S. Pat. Nos. 4,112,183 and 4,254,351 (Smith and Smith et al.), respectively. In many cases, mica tape is wrapped around the coil and then impregnated with low viscosity liquid insulation resin by vacuum-pressure impregnation ("VPI"). That process consists of evacuating a chamber containing the coil in order to remove air and moisture trapped in the mica tape, then introducing the insulation resin under pressure to impregnate the mica tape completely with resin thus eliminating voids, producing resinous insulation in a mica matrix. This resin is subsequently cured by a prolonged heating cycle. In practice, complete elimination of voids is difficult, and the voids can be a recurring source of electrical and mechanical problems. And of course, the mica tape is thick, bulky, and difficult to apply to the coils.
Problems with mica as presently used occur in two areas: (1) microscopically, at the interface between the mica and polymeric insulation, and (2) in the VPI process required to fill the mica tape layers completely with polymeric insulation. The mica surface is a problem area because it is not "wet" very well by the insulation resin. Thus, there is a tendency for voids to form at the mica surface that are not completely eliminated during evacuation of the coil prior to impregnation with the insulation resin. Surface treatments of the mica or addition of wetting agents to the resin have not completely eliminated this problem to date. These voids can have significant consequences for both the electrical performance of the coil and its mechanical integrity. Electrically the voids can act as locations for partial discharges, which increase the electrical losses in the coil and can degrade the surrounding insulation during prolonged exposure. Mechanically the voids can be places where delamination can begin, causing potential disintegration of the coil.
The problems associated with the VPI process are primarily the result of the several steps involved: (1) bake out of the coil, (2) evacuation, (3) impregnation, and (4) curing. Each step is time-consuming and must be carried out correctly in order to produce a finished coil which meets the electrical and mechanical requirements. The process time and scrap coils represent significant increased cost of the coil fabrication method.
The requirement of using mica for high voltage insulation has been questioned. Bjorklund et al., of A.B.B., in "A New Mica-Free Turn Insulation For Rotating HV Machines," the Conference Record of the 1994 IEEE International Symposium on Electrical Insulation, June 5-8, 1994 pp. 482-484, taught use of a chromium oxide protective layer for a resin enamel as copper turn insulation, which was then and easily manufactured, as a substitute for arramed paper containing 50% mica. The nonlinearity of the chromium oxide apparently has a large impact on the absorption of free electron charges.
Others had previously experimented with highly positive charged materials having good thermal stability. Drljaca et al. in "Intercalation of Montmorillonite with Individual Chromium (III) Hydrolytic Oligomers", Vol. 31, No. 23, 1992, pp. 4894-4897, taught chromium inserted/intercalated pillared clays as having sorptive and catalytic properties and possible substitutes for zeolites, that is, sodium or calcium aluminosilicates used for ion exchange water softening. Drljaca et al. further described, in "A New Method for Generating Chromium (III) Intercalated Clays," Inorganica Chimica Acta, 256, 1997, pp. 151-154, Cr (III) dimer reaction with other dimer units to form planar sheets for intercalation into montmorillonite clays, Al.sub.2 O.sub.3.4SiO.sub.2.H.sub.2 O
In a different area, though still related to clays, Miller, in "Tiny Clay Particles Pack Patent Properties Punch," Plastics World, Fillers, October 1997, pp. 36-38, describes mineral filled plastic nanocomposites having excellent mechanical strength, heat resistance, flame retardancy and gas-barrier properties. These composites originally used nylon materials containing bundles of small platelets of montmorillonite clay, about 0.5 micrometer to 2 micrometers wide and 1 nanomeler (nm) thick, that is, 0.001 micrometer thick, for automobile timing belts. More recently, attempts have been made to incorporate such platelets into other resins. Miller further describes the platelets as having a high "aspect ratio," that is, high width compared to thickness, where molecular bonds are formed between the platelets and a polymer during compounding. The clay producers, such as Nancor Inc. and AMCOL Intl., chemically stretch, that is, "open" the spacing between the platelets from about 4 Angstrom Units, about 0.0004 micrometer, to a thickness such that organic resin molecules can directly ionically or covalently attach to the platelet surface, allowing the platelet to directly react into the polymer structure during subsequent polymerization/compounding. The platelet bundles are also exfoliated into individual platelets by the clay producers to aid in polymerization/compounding. The molecular "tail", Miller states, has the chemical functionality to overcome the incompatibility between the hydrophilic (having an affinity for water) clay and the hydrophobic (water-repelling) organic polymer and enable them to directly form a molecular bond, that is, directly intercalate the polymer into the nanoclay. Besides timing belts, additional uses appear to be thermoplastic resin gas barrier packaging, microwavable containers, and epoxy resin circuit boards.
These processes are also generally described by Usuki et al., of Toyota Chou, in U.S. Pat. No. 4,889,885. There, onium ions, from materials such as ammonium salts, sulfonium salts and phosphonium salts, were used to expand the interlayer distance of a clay such as montmorillonite through ion exchange with inorganic ions in the clay mineral. This permits the clay mineral to take a polymer into the interlayer space and connects the layers of clay mineral and polymer directly to each other through ionic bonds. The onium salt has a molecular skeleton which becomes the polymerization initiator. In cases where the onium salt has a molecular skeleton which becomes the basic constituting unit of the resin, the salt will have a phenol group (for phenolic resin), an epoxy group (for epoxy resin) and a polybutadiene group (for acrylonitrilebutadienerubber). Yano and Usuki et al. of Toyota R&D, in "Synthesis and Properties of Polyamide--Clay Hybrid", Journal of Polymer Science, Part A, Polymer Chemistry, Vol. 31, 1993, pp. 2493-2498, describe use of montmorillonite intercalated with an ammonium salt of dodecylamine as an aligned filler in a polyamide resin hybrid, for use as a gas barrier film. There, it appears a sodium type montmorillonite was mixed with hot water to disperse the sodium, which was then replaced with the ammonium salt of dodecylamine which then interacted with dimethylacetamide ("DMAC") to "open" the platelets of montmorillonite. The intercalated montmorillonite was then simply dispersed into a polyamide matrix and cast as a film, where the montmorillonite oriented parallel to the film surface to provide barriers to gas permeation.
The exfoliation and polymer intercalation of platelet bundles is also described in U.S. Pat. No. 5,698,624 (Beall et al.) where polymerizable monomers are directly intercalated between platelets or admixed with exfoliated material and then polymerized. Suitable polymers taught are polyamides, polyesters, polyurethanes and polyepoxides among others. Here organic ammonium molecules are inserted into sodium or calcium montmorillonite clay platelets to increase the thickness within the platelets, "open", followed by high shear mixing to exfoliate the silicate layers which are then directly mixed with a matrix polymer to improve mechanical strength and/or high temperature characteristics. All instances of polymer interaction with the platelets appear to be direct interaction between the polymer and the "opened" nanoplatelet. Other patents in this area include U.S. Pat. Nos. 5,721,306; 5,760,121; and 5,804,613 (Tsipursky et al.; Beall et al.; and Beall et al. respectively).
While impregnated and vacuum pressure impregnated mica tape remain the standard for high voltage electrical insulation and chromium oxide overcoats prove enhanced PD (partial discharge) resistance, the need still exists for ultra thin low cost high voltage electrical insulation that can be dip coated, sprayed or extruded on high voltage electrical conductors in one application yet will have all the desirable characteristics of the bulky mica matrix insulation.